PSI CAP (Psi Capricorni). The classical figure of Capricornus, the Water Goat,
bottoms out to the south with the fourth magnitude red giant Omega Capricorni. Just two degrees to the
northwest of it lies just-barely-fainter (magnitude 4.14) Psi Cap,
whose name lies almost as far down the Greek
alphabet. Other than line-of-sight, there is no relation
between the two. Psi Cap is a rather local Sunlike class F (F4)
dwarf with a distance of only 47.9 light years, while Omega is more
than 10 times farther away, testimony to the relative faintness of
Psi and the grandeur of Omega. With a temperature of 6620 Kelvin,
Psi Cap is just 840 degrees warmer than our own Sun. With little infrared or ultraviolet to
account for, the luminosity comes in at just 3.8 times solar, which
leads to a mass of 1.35 times that of the Sun. Still fairly
youthful, the star is about a third to half of the way through its
four billion year hydrogen-fusing lifetime. At first EVERYTHING
seems almost solar, even the radius of just 1.5 times that of the
Sun and the near-solar metal content. But then come the
differences. With a projected equatorial spin-speed of 39
kilometers per second, Psi Cap lies right at the so-called
"rotation break," the point at which (going from cool to hot)
rotation speeds suddenly increase as a result of a the drop-off in
the magnetic fields caused by convective circulation. (The winds
from cooler stars drag out the stellar magnetic fields that then
act like ropes that drag back and act like stellar brakes.) Given
the radius, the star then rotates in under 1.9 days. Rotation
indeed seems to be Psi Cap's strong point, as it is the first star
whose "differential rotation" has been observed through the
appearance of its spectrum, the star (like the Sun) taking longer
to rotate at higher latitudes than lower. Such rotation is in part
responsible for magnetic activity, and indeed Psi Cap shows it.
Mid-temperature solar-type dwarfs are of course known for their planets. Psi Cap seems not to have any, rather
it does not have the kind of debris disk that is left over after
their creation and that is fueled by planetary collisions. Nor
does it have any recognized companion, the star seemingly
quite alone.